This invention relates to radar testing systems, and is specifically concerned with both a system and a method for broadcasting a target simulating signal from a remotely located horn antenna that utilizes a laser-powered optical link to reduce power losses between a test target generator located near the radar system being tested, and the horn antenna.
Devices for testing military radar systems are known in the prior art. Such systems operate by providing a target simulating signal to the receiver of the radar system whose shape, frequency and phase angle simulates a moving target characterized by a distinctive signature that indicates what the target might be (i.e., helicopter, airplane or ship). Such target simulating devices are very useful in testing and calibrating military radar systems, as they allow the system operator to experience the radar system's response to a variety of types of targets under a variety of different speeds and distances without the need for providing drone targets, which is both inconvenient and expensive.
One of the first target simulating devices that was developed in the prior art was the repeater-modulator test apparatus. This device included a receiver antenna for collecting a sample of the pulses emitted by the transmitter of the radar system, in combination with tuned oscillator circuits for generating a target simulating signal that was recognizable as such by the receiver of the radar system. Such repeater-modulators also included various frequency, phase shifting and amplifier circuits for imparting various frequency, phase and amplitude characteristics to the resulting signal which the radar receiver would in turn interpret as speed, distance and target signature characteristics.
While such repeater-modulators have proven to be an effective means for testing a military radar system, there are unfortunately a number of drawbacks associated with such devices. For example, because the oscillator circuits that generate the target simulating signal emitted by these devices have to be specifically tuned to the transmission frequencies of the radar system being tested, repeater-modulators are narrow-band, custom-made devices that cannot easily be adapted to test more than one particular radar system. The dedicated nature of these devices, in combination with the sophisticated precision electronics which they must necessarily employ, renders them quite expensive. Still another shortcoming resides in the fact that the radar system being testing must have a complete and operable radar transmitter capable of transmitting target seeking pulses for this type of device to be used. This is a significant drawback, as the transmitter of such radar systems is usually the last major component of the radar system to be built and rendered operable. Thus the system cannot be effectively tested at an intermediate stage of construction, when only the radar receiver is operable. The necessity of transmitting actual target seeking pulses from the radar transmitter also poses a safety hazard to the personnel conducting the test, as such pulses at short range constitutes a potentially dangerous radiation source. Finally, the need to transmit actual target seeking pulses during the testing phase of the system poses a security hazard, as these pulses can be intercepted by hostile countries having an interest in the precise frequencies and characteristics of the radar systems used by the armed forces of the United States.
To overcome the shortcomings of repeater-modulation radar testing devices, RF horn antenna testing systems were developed. These systems have generally comprised a test target generating circuit whose input is slaved to the signal generating oscillators of the radar system, and a horn antenna remotely positioned from the receiver of the radar system for emitting the signal generated by the test target generator. Unlike repeater-modulator radar testing devices, such RF horn antenna testing systems are relatively broad-band devices which are able to generate the frequency specific pulses characteristic of a particular radar system being tested by slaving, rather than by custom tuning. Hence horn antenna testing systems are easily adapted for use on a variety of different radar systems. Since these systems do not require the transmitter of the radar system being tested to operate, but only the receiver thereof, testing can be commenced before the transmitter of the radar system becomes operational. The testing can also be accomplished in a safer and more secure manner, since it is unnecessary for the radar transmitter to emit any radiation that is potentially hazardous to the testing personnel, and potentially monitorable by hostile nations. This system has the added advantage of being simpler and less expensive than the previously described repeater-modulator testing devices.
However, despite these advantages, RF horn antenna target simulating systems also have drawbacks, the most major of which is the necessity of booster amplifiers along the length of the coaxial cable running from the target signal generator to the horn antenna. These losses are a consequence of the high impedances that coaxial cables exert on high frequency RF, in combination with the inherent limitations associated with the minimum distance that the horn antenna can be placed with respect to the receiver of the radar system in order for valid and accurate testing to be carried out. In order for the target simulating signals emitted by any type of testing system to appear as "point source targets" to the receiver of the radar system, the horn antennas have to be placed past what is known as the "near field" or Rayleigh field of the radar system being tested. Otherwise, the signals which they emanate will not appear in focus, and may even appear up to three times their actual size due to the non-parallel wave front of the target simulating pulse at short distances from the targets. Hence, if the tests are to be conducted accurately, it is essential that the horn antenna be placed outside the near field, which may be computed as follows: EQU R=2D.sup.2 /L
where
R is the minimum range PA1 D=diameter or width of the radar receiver antenna PA1 L=the free space transmit wavelength
For X-band radars currently in production, the minimum range R can be an excess of 500 ft. Because the coaxial cable which connects the test target generating circuit with the horn antenna typically attenuates RF at approximately 0.25 to 0.75 decibels per foot, over 250 dB worth of amplification would have to be provided to compensate for these power losses. Typically, booster amplifiers must be provided at various junctions in the coaxial cable along its 500 foot or greater length to obviate the need for a very large amplifier in the vicinity of the horn antenna. Of course, the use of one or more of these amplifiers could be obviated by providing a more powerful test target generating circuit. However, because the slaving connection between the test target generating circuit and the transmitter of the radar system requires the generator to be relatively close to the radar receiver, and because a generating circuit which operates at a power lever of much greater than 20 watts will generate a high enough level of radiation to interfere with the receiver of the radar system being tested, the use of such a high-power test target generating to obviate the use of booster amplifiers would require a substantial amount of expensive RF shielding. To overcome the limitations associated with coaxial cable power losses, modified designs of the RF horn antenna testing system have been developed wherein a pipe-like waveguide is used in lieu of a coaxial cable, since such waveguides have much better transmission efficiencies. However, these systems suffer from the additional expense associated with laying over 500 feet of a rigid, pipe-like waveguide. Moreover, a great deal of flexibility in moving the target horn in the test field is lost, since it is virtually impossible to move the waveguide once it is laid.
Clearly, a radar testing system is needed that preserves all of the advantages associated with RF horn antenna systems, but which does not suffer the power loss limitations associated with coaxial cable which require the use of booster amplifiers, or the rigidity and lack of versatility associated with the laying of a pipe-like waveguide over very long distances.